Diaper providing temperature insensitive liquid handling

ABSTRACT

An acquisition member for use in disposable absorbent articles. The acquisition member including a multitude of fibers and a latex binder and having a void volume of at least 7 cubic centimeter per gram in the temperature range from 20° C. to 40° C. The latex binder may be a dispersion of a polymer in an essentially aqueous medium. The polymer is capable of forming a film having a tan δ value at 40° C. that is not greater than its tan δ value at 20° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/912,991filed Aug. 6, 2004 now U.S. Pat. No. 7,786,341.

FIELD OF THE INVENTION

The present invention relates to absorbent articles such as disposablediapers, sanitary napkins and panty liners. More specifically, thepresent invention relates to acquisition layers and materials for suchacquisition layers, which are very suitable to be used with thinproducts comprising a relatively high concentration of super-absorbentpolymer material.

BACKGROUND OF THE INVENTION

Disposable absorbent articles are broadly available and consumers areused to a high performance for the collecting and retaining of menses(in the case of sanitary napkins or panty liners) or for the collectingand retaining urine and fecal material (in the case of e.g. disposablediapers). However, consumers do not only expect a superior absorbencybehavior, but place more and more emphasis on the wearing comfort ofsuch articles, and namely on the dryness of those articles.

Typically, such articles comprise multiple absorbent members, at leastone member being primarily designed to store liquid, and at least oneother member primarily designed to acquire and/or distribute liquid.

At least the storage member will often comprise super-absorbentmaterial, which is admixed with the traditionally used pulp fibermaterial. Such super-absorbent materials can absorb many times (e.g. 10,20 or 30 times) their own weight and are therefore very helpful whendesigning an article of improved fluid handling properties. Many recentproducts employ higher and higher concentrations of super-absorbentmaterials, that is concentrations in excess of 50% of the total weightof the storage member. These products achieve a high absorbing capacitywith a very thin storage member and are thereby typically overall thinproducts. While super-absorbent materials can store very large amountsof liquid, they are often not able to distribute the liquid from thepoint of impact to more remote areas of the absorbent article and toacquire the liquid as fast as it may be received by the article.

For this reason acquisition members are used, which provide for theinterim acquisition of large amounts of liquid and which often alsoallow for the distribution of liquid. Thereby the acquisition memberplays a key role in using the whole absorbent capacity provided by thestorage member.

An exemplary state of the art absorbent article, namely a sanitarynapkin, is disclosed in WO 00/51651. The so-called “secondary topsheet”primarily provides for the liquid acquisition and distribution. Thestorage member is provided in the form of a core that can contain a highportion of super-absorbent polymer material.

State of the art absorbent articles in the form of disposable diapersare disclosed in the following co-pending U.S. application Ser. Nos.10/423,425 and 10/776,851. The storage member of these articles maycontain more than 50 weight-percent of super-absorbent polymer material,even more than 80% and in some embodiments even about 100% of thesuper-absorbent polymer material. While these articles are designed tohave excellent fluid handling and fluid storage behavior, they maysometimes not be as comfortable to wear as some consumers would like it,namely those consumers interested in a premium product.

For achieving excellent fluid handling and fluid storage properties therespective storage members use super-absorbent materials, which areselected mostly with an emphasis on fluid handling properties. Forexample EP 304319 B1 (Goldman et al.) discloses the benefits of arelatively narrow distribution of the particle size of thesuper-absorbent polymers. The respective articles will preferably notcomprise many fines, but relatively coarse particles. EP 752892 B1(Goldman et al.) discloses an absorbent structure, which usesconcentrations of super-absorbent particles in access of 60% and teachesto use super-absorbent particles of a relatively high porosity.

Materials suitable to meet the above outlined requirements for a liquidacquisition member must meet these requirements not only in standard orideal conditions, but in a variety of conditions, namely at differenttemperatures and pressures.

Those different temperatures and pressures are encountered, for example,under the in-use conditions. In use, the temperature at which theacquisition layer must operate depends both on the body temperature ofthe wearer and on the ambient temperature (which is typically below thebody temperature, but in some climates can also exceed the bodytemperature). Most pressure on the liquid acquisition layer is typicallyexerted when a wearer is sitting. The temperature depends on the bodyweight of the wearer but can also largely depend on the surface on whichthe wearer sits.

Other relevant conditions regarding the performance of liquidacquisition layer are those conditions of storage and transport. In oneaspect, this includes the conditions under which the diapers onceproduced are stored and transported. Typically a diaper package exertssome compressive force onto the diaper and, depending largely on thecountry of marketing, may undergo a rather large range of temperatures,before being delivered to the consumer. It is important, that the liquidacquisition layer irrespective of its storage and transport historyrecovers to a condition, which allows for optimal liquid handling. In asecond aspect, the transport and storage conditions of the material tobe formed into a liquid acquisition layer before the manufacturing ofthe diaper are also relevant. Very often, these materials are providedas roll stock materials. To allow for economic handling and transportthe material is wound onto a roll under some pressure. The respectiverolls my also be stored over a relatively long period of time, and whilebeing stored or transported be exposed to a large variety of ambienttemperatures. Of course, irrespective also of the storage and transporthistory a material may have undergone in these earlier stages of itslifetime, a material must recover, such that optimal liquid handlingperformance is achieved when the material is ultimately used as a liquidacquisition layer and expected to perform in a diaper.

U.S. Pat. No. 5,997,980 (Matoba et al.) discloses the use of hollowpolyester fibers for providing a nonwoven material, the hollow polyesterfibers having a relatively large hollow volume of 40% to 85%. Thematerial is said to have a high resistance to compression and a highrecovery from compression.

EP 1 032 345 B1 discloses a fluid acquisition-transfer layer for anabsorbent article which comprises a mixture of heat-shrunk, spiralized,thermoplastic multi-component, preferably, bi-component, functionalfibers. WO 03/048440 (Brown et al.) discloses the use of similar fibersfor a non-woven fabric for a use in personal care absorbent articles,the fibers namely being single polymer, helically crimped fibers.

WO 98/22279 (Flohr et al.) discloses acquisition layers for improvedliquid handling in absorbent articles. As a preferred fluid handlingmaterial fibrous materials, which are preferably carded are disclosed.These fibers are chemically bonded by a resin. The disclosed resins aremore hydrophilic than the base polymer and are non-uniformly distributedin the fluid handling material throughout the thickness (z-direction).The fluid handling material is designed to achieve an acquisitionperformance of less than 2 seconds for the third impacting gush ofliquid. The disclosed acquisition layer materials are disclosed as beingeasy to handle and easy to transport without requiring a large volume.

EP 149 880 (Kwok) discloses non-woven webs of synthetic fibersconsolidated by means of carboxylated styrene-butadiene lattices, anddisposable articles make therefrom. The obtained non-wovens aredescribed as having improved wet tensile and elongation properties.

US 2003/0105190 (Diehl et al.) discloses a latex for utilization as abinder for nonwoven applications and a process for preparing such latex.The latex is essentially of the styrene-butadiene type.

The prior art comprises many examples of attempts to improve therheology of adhesives, typically hot-melt adhesives and also hydrogels,allowing for better application of the adhesive materials or for betteradhesion. For example, WO 98/28022 (P. Coles et al.) discloses topicaladhesive for the application of functional articles. The adhesive has anelastic modulus G′ at a temperature of 37° C. and a viscous modulus G″at 37° C. The patent application discloses optimal ranges andrelationships for G′ and G″. WO 02/78756 discloses a hydrogel adhesivecomprising a weak-acid monomer, and having an elastic modulus G′ at 25°C. in the range of 2000 pa to 6000 pa.

It is one objective of the present invention to provide an absorbentarticle, which has improved liquid handling characteristics as comparedto the above disclosed articles.

Moreover, it is an objective of the present invention to provide fluidacquisition layers for improved liquid handling characteristics, namelyfor use with storage layer comprising a high concentration of superabsorbent polymers.

In one further important aspect it is an objective of the presentinvention to provide an article, which is more comfortable to wear, andwhich in particular provides superior dryness.

SUMMARY OF THE INVENTION

The present invention relates to absorbent articles, such as diapers andsanitary napkins, and acquisition members useful for such articles. Morespecifically, the invention relates to an acquisition member comprisinga multitude of fibers and a latex binder, the liquid acquisition memberhaving a void volume of at least 7 cubic centimeter per gram in thetemperature range from 20° C. to 40° C., the binder comprising adispersion of a polymer substance in an essentially aqueous medium, thepolymer substance being capable of forming a film, said film having atan δ value, as defined herein, at 20° C. and at 40° C., wherein the tanδ value at 40° C. is not greater than said tan δ value at 20° C.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out anddistinctly claiming the present invention, it is believed the same willbe better understood by the following drawings taken in conjunction withthe accompanying specification wherein like components are given thesame reference number.

FIG. 1 is a top plan view of a disposable diaper, with the upper layerspartially cut away.

FIG. 2 is a cross-sectional view of the disposable diaper shown in FIG.1.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

As used herein, the following terms have the following meanings:

“Absorbent article” refers to devices that absorb and contain liquid,and more specifically, refers to devices that are placed against or inproximity to the body of the wearer to absorb and contain the variousexudates discharged from the body. Absorbent articles include but arenot limited to diapers, adult incontinent briefs, training pants, diaperholders and liners, sanitary napkins and the like. Absorbent articlesalso include wipes, such as household cleaning wipes, baby wipes, andthe like.

“Disposable” is used herein to describe articles that are generally notintended to be laundered or otherwise restored or reused i.e., they areintended to be discarded after a single use and, preferably, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner.

“Disposed” is used to mean that an element(s) is formed (joined andpositioned) in a particular place or position as a unitary structurewith other elements or as a separate element joined to another element.

“Diaper” refers to an absorbent article generally worn by infants andincontinent persons about the lower torso.

The terms “thickness” and “caliper” are used herein interchangeably.

“Attached” or “Joined” encompasses configurations whereby an element isdirectly secured to another element by affixing the element directly tothe other element, and configurations whereby an element is indirectlysecured to another element by affixing the element to intermediatemember(s) which in turn are affixed to the other element.

“Comprise,” “comprising,” and “comprises” is an open ended term thatspecifies the presence of what follows e.g. a component but does notpreclude the presents of other features, elements, steps or componentsknown in the art, or disclosed herein.

The term “hydrophilic” describes fibers or surfaces of fibers, which arewettable by aqueous fluids (e.g. aqueous body fluids) deposited on thesefibers. Hydrophilicity and wettability are typically defined in terms ofcontact angle and the strike through time of the fluids, for examplethrough a nonwoven fabric. This is discussed in detail in the AmericanChemical Society publication entitled “Contact angle, wettability andadhesion”, edited by Robert F. Gould (Copyright 1964). A fiber orsurface of a fiber is said to be wetted by a fluid (i.e. hydrophilic)when either the contact angle between the fluid and the fiber, or itssurface, is less than 90°, or when the fluid tends to spreadspontaneously across the surface of the fiber, both conditions arenormally co-existing. Conversely, a fiber or surface of the fiber isconsidered to be hydrophobic if the contact angle is greater than 90°and the fluid does not spread spontaneously across the surface of thefiber.

The terms “fiber” and “filament” are used interchangeably.

The terms “nonwoven”, “nonwoven fabric” and “nonwoven web” are usedinterchangeable.

Absorbent Articles

FIG. 1 is a plan view of a diaper 20 as a preferred embodiment of anabsorbent article according to the present invention. The diaper isshown in its flat out, uncontracted state (i.e., without elastic inducedcontraction). Portions of the structure are cut away to more clearlyshow the underlying structure of the diaper 20. The portion of thediaper 20 that contacts a wearer is facing the viewer. The chassis 22 ofthe diaper 20 in FIG. 1 comprises the main body of the diaper 20. Thechassis 22 comprises an outer covering including a liquid pervioustopsheet 24 and/or a liquid impervious backsheet 26. The chassis mayalso include most or all of the absorbent core 28 encased between thetopsheet 24 and the backsheet 26. The chassis preferably furtherincludes side panels 30, leg cuffs 32 and a waist feature 34. The legcuffs and the waist feature typically comprise elastic members 33. Oneend portion of the diaper 20 is configured as the front waist region 36of the diaper 20. The opposite end portion is configured as the rearwaist region 38 of the diaper 20. An intermediate portion of the diaper20 is configured as the crotch region 37, which extends longitudinallybetween the front and rear waist regions 36 and 38. The crotch region 37is that portion of the diaper 20 which, when the diaper 20 is worn, isgenerally positioned between the wearer's legs. The waist regions 36 and38 may include a fastening system comprising fastening members 40preferably attached to the rear waist region 38 and a landing zone 42attached to the front waist region 36. The diaper 20 has a longitudinalaxis 100 and a transverse axis 110. The periphery of the diaper 20 isdefined by the outer edges of the diaper 20 in which the longitudinaledges 44 run generally parallel to the longitudinal axis 100 of thediaper 20 and the end edges 46 run generally parallel to the transverseaxis 110 of the diaper 20.

For unitary absorbent articles, the chassis 22 comprises the mainstructure of the diaper with other features added to form the compositediaper structure. While the topsheet 24, the backsheet 26, and theabsorbent core 28 may be assembled in a variety of well-knownconfigurations, preferred diaper configurations are described generallyin U.S. Pat. No. 5,569,234 entitled “Disposable Pull-On Pant” issued toBuell et al. on Oct. 29, 1996; and U.S. Pat. No. 6,004,306 entitled“Absorbent Article With Multi-Directional Extensible Side Panels” issuedto Robles et al. on Dec. 21, 1999.

The topsheet 24 in FIG. 1 may be fully or partially elasticized or maybe foreshortened to provide a void space between the topsheet 24 and theabsorbent core 28. Exemplary structures including elasticized orforeshortened topsheets are described in more detail in U.S. Pat. No.5,037,416 entitled “Disposable Absorbent Article Having ElasticallyExtensible Topsheet” issued to Allen et al. on Aug. 6, 1991; and U.S.Pat. No. 5,269,775 entitled “Trisection Topsheets for DisposableAbsorbent Articles and Disposable Absorbent Articles Having SuchTrisection Topsheets” issued to Freeland et al. on Dec. 14, 1993.

The backsheet 26 in FIG. 1 is generally the portion of the diaper 20positioned with the absorbent core 28 between the backsheet 26 and thetopsheet 24. The backsheet 26 may be joined with the topsheet 24. Thebacksheet 26 prevents the exudates absorbed by the absorbent core 28 andcontained within the article 20 from soiling other external articlesthat may contact the diaper 20, such as bed sheets and undergarments. Inpreferred embodiments, the backsheet 26 is substantially impervious toliquids (e.g., urine) and comprises a laminate of a nonwoven and a thinplastic film such as a thermoplastic film having a thickness of about0.012 mm (0.5 mil) to about 0.051 mm (2.0 mils). Suitable backsheetfilms include those manufactured by Tredegar Industries Inc. of TerreHaute, Ind. and sold under the trade names X15306, X10962, and X10964.Other suitable backsheet materials may include breathable materials thatpermit vapors to escape from the diaper 20 while still preventingexudates from passing through the backsheet 26. Exemplary breathablematerials may include materials such as woven webs, nonwoven webs,composite materials such as film-coated nonwoven webs, and microporousfilms such as manufactured by Mitsui Toatsu Co., of Japan under thedesignation ESPOIR NO and by EXXON Chemical Co., of Bay City, Tex.,under the designation EXXAIRE.

The absorbent core 28 in FIG. 1 generally is disposed between thetopsheet 24 and the backsheet 26. The absorbent core 28 may comprise anyabsorbent material that is generally compressible, conformable,non-irritating to the wearer's skin, and capable of absorbing andretaining liquids such as urine and other certain body exudates. Theabsorbent core 28 may comprise a wide variety of liquid-absorbentmaterials commonly used in disposable diapers and other absorbentarticles such as comminuted wood pulp, which is generally referred to asair felt. Examples of other suitable absorbent materials include crepedcellulose wadding; melt blown polymers, including co-form; chemicallystiffened, modified or cross-linked cellulosic fibers; tissue, includingtissue wraps and tissue laminates, absorbent foams, absorbent sponges,superabsorbent polymers, absorbent gelling materials, or any other knownabsorbent material or combinations of materials. The absorbent core mayfurther comprise minor amounts (typically less than 10%) of non-liquidabsorbent materials, such as adhesives, waxes, oils and the like.

Exemplary absorbent structures for use as the absorbent assemblies aredescribed in U.S. Pat. No. 4,834,735, entitled “High Density AbsorbentMembers Having Lower Density and Lower Basis Weight Acquisition Zones”,issued to Alemany et al. on May 30, 1989; and U.S. Pat. No. 5,625,222entitled “Absorbent Foam Materials For Aqueous Fluids Made From highInternal Phase Emulsions Having Very High Water-To-Oil Ratios” issued toDesMarais et al. on Jul. 22, 1997.

The diaper 20 may also include such other features as are known in theart including front and rear ear panels, waist cap features, elasticsand the like to provide better fit, containment and aestheticcharacteristics. Such additional features are well known in the art andare described in U.S. Pat. No. 3,860,003 entitled “Contractable sideportions for disposable diaper” issued to Buell et al. on Jan. 14, 1975and U.S. Pat. No. 5,151,092 entitled “Absorbent article with dynamicelastic waist feature having a predisposed resilient flexural hinge”issued to Buell et al. on Sep. 29, 1992.

In order to keep the diaper 20 in place about the wearer, the waistregions 36 and 38 may include a fastening system comprising fasteningmembers 40 preferably attached to the rear waist region 38. In apreferred embodiment the fastening system further comprises a landingzone 42 attached to the front waist region 36. The fastening member isattached to the front waist region 36, preferably to the landing zone 42to form leg openings and an article waist.

Diapers 20 according to the present invention may be provided with are-closable fastening system or may alternatively be provided in theform of pant-type diapers.

The fastening system and any component thereof may include any materialsuitable for such a use, including but not limited to plastics, films,foams, nonwoven webs, woven webs, paper, laminates, fiber reinforcedplastics and the like, or combinations thereof. It may be preferablethat the materials making up the fastening device be flexible. Theflexibility is designed to allow the fastening system to conform to theshape of the body and thus, reduces the likelihood that the fasteningsystem will irritate or injure the wearer's skin.

FIG. 2 shows a cross-sectional view of FIG. 1 taken in the transverseaxis 110. Starting from the wearer facing side the diaper comprises thetopsheet 24, the components of the absorbent core 28, and the backsheet26. The absorbent core preferably comprises an acquisition system 50,which comprises an upper acquisition layer 52 facing towards the wearerand a lower acquisition layer 54. In one preferred embodiment the upperacquisition layer comprises a nonwoven fabric whereas the loweracquisition layer preferably comprises a mixture of chemicallystiffened, twisted and curled fibers, high surface area fibers andthermoplastic binding fibers. In another preferred embodiment bothacquisition layers are provided from a non-woven material, which ispreferably hydrophilic. The acquisition layer preferably is in directcontact with the storage layer 60.

Acquisition members according to the present invention are preferablycomprised by the acquisition system 50 of a diaper as described. Mostpreferably, the acquisition members are comprised by the upperacquisition layer 52. In one preferred embodiment of the presentinvention the upper acquisition layer 52 consists of the claimedacquisition member.

The storage layer 60 is preferably wrapped by a core wrap material. Inone preferred embodiment the core wrap material comprises a top layer 56and a bottom layer 58. The top layer 56 and the bottom layer 58 can beprovided from a non-woven material. One preferred material is aso-called SMS material, comprising a spunbonded, a melt-blown and afurther spunbonded layers. The top layer 56 and the bottom layer 58 maybe provided from two or more separate sheets of materials or they may bealternatively provided from a unitary sheet of material. Such a unitarysheet of material may be wrapped around the storage layer 60, e.g. in aC-fold. The top layer 56 and the bottom layer 58 may also be joined toeach other, preferably along their periphery. In one preferred optionboth layers are joined along their longitudinal peripheries, in otherembodiments they are joined along the transversal peripheries, or alongthe longitudinal and the transversal peripheries. The joining can beachieved my multiple means well known in the art, eg. by adhesive means,using a continuous or a discontinuous pattern, and preferably a linearor curvilinear pattern.

The storage layer 60 typically comprises fibrous materials, mixed withsuperabsorbent, absorbent gelling materials. Other materials describedabove as suitable for the absorbent core 28 may also be comprised.

Nonwoven Fabrics

A nonwoven fabric is a manufactured sheet, web or batt of directionallyor randomly orientated fibers, bonded by friction, and/or cohesionand/or adhesion, excluding paper and products which are woven, knitted,tufted, stitch-bonded incorporating binding yarns or filaments, orfelted by wet-milling, whether or not additionally needled.

The fibres may be of natural or man-made origin. They may be staple orcontinuous filaments or be formed in situ.

Nonwoven fabrics can be formed by many processes such as meltblowing,spunbonding, carded. The basis weight of nonwoven fabrics is usuallyexpressed in grams per square meter (gsm).

Commercially available fibers have diameters ranging from less thanabout 0.001 mm to more than about 0.2 mm and they come in severaldifferent forms: short fibers (known as staple, or chopped), continuoussingle fibers (filaments or monofilaments), untwisted bundles ofcontinuous filaments (tow), and twisted bundles of continuous filaments(yarn). Fibers are classified according to their origin, chemicalstructure, or both. They can be braided into ropes and cordage, madeinto felts (also called nonwovens or non-woven fabrics), woven orknitted into textile fabrics, or, in the case of high-strength fibers,used as reinforcements in composites—that is, products made of two ormore different materials.

The nonwoven fabrics may comprise fibers made by nature (naturalfibers), made by man (synthetic or man-made), or combinations thereof.Example natural fibers include but are not limited to: animal fiberssuch as wool, silk, fur, and hair; vegetable fibers such as cellulose,cotton, flax, linen, and hemp; and certain naturally occurring mineralfibers. Synthetic fibers can be derived from natural fibers or not.Example synthetic fibers, which are derived from natural fibers includebut are not limited to rayon and lyocell, both of which are derived fromcellulose, a natural polysaccharide fiber. Synthetic fibers, which arenot derived from natural fibers can be derived from other naturalsources or from mineral sources. Example synthetic fibers not derivedfrom natural sources include but are not limited to polysaccharides suchas starch. Example fibers from mineral sources include but are notlimited to polyolefin fibers such as polypropylene, polyethylene fibersand polyester, which are derived from petroleum, and silicate fiberssuch as glass and asbestos.

Nonwoven webs can be formed by direct extrusion processes during whichthe fibers and webs are formed at about the same point in time, or bypreformed fibers, which can be laid into webs at a distinctly subsequentpoint in time. Example direct extrusion processes include but are notlimited to: spunbonding, meltblowing, solvent spinning, electrospinning,and combinations thereof typically forming layers.

Example “laying” processes include wetlaying and drylaying. Exampledrylaying processes include but are not limited to airlaying, carding,and combinations thereof typically forming layers. Combinations of theabove processes yield nonwovens commonly called hybrids or composites.Example combinations include but are not limited tospunbond-meltblown-spunbond (SMS), spunbond-carded (SC),spunbond-airlaid (SA), meltblown-airlaid (MA), and combinations thereof,typically in layers. Combinations which include direct extrusion can becombined at about the same point in time as the direct extrusion process(e.g., spinform and coform for SA and MA), or at a subsequent point intime. In the above examples, one or more individual layers can becreated by each process. For instance, SMS can mean a three layer, ‘sms’web, a five layer ‘ssmms’ web, or any reasonable variation thereofwherein the lower case letters designate individual layers and the uppercase letters designate the compilation of similar, adjacent layers.

The fibers in a nonwoven web are typically joined to one or moreadjacent fibers at some of the overlapping junctions. This includesjoining fibers within each layer and joining fibers between layers whenthere is more than one layer. Fibers can be joined by mechanicalentanglement, by chemical bond or by combinations thereof. Fibers canalso be joined by heat-bonding, which comprises techniques such asthrough-air bonding and thermobonding by use of heated calendar rolls.

All of the above described fibers and manufacturing techniques can beuseful for providing an acquisition member according to the presentinvention.

Preferred Acquisition Members

Liquid acquisition members in accordance with the present invention willachieve a certain minimal void volume, namely at least 7 cubiccentimeterper gram. Many materials comprising a multitude of fibers and a binder,namely many non-woven material as described above are suitable forproviding a liquid acquisition member according to the presentinvention. Preferably the respective liquid acquisition members willachieve a void volume value of at least 9 cubiccentimeter per gram, morepreferably 11 cubiccentimeter per gram, and most preferablycubiccentimeter per gram per gram. A material with a high void volume isa material, which is not dense, and hence which provides enough voidvolume for efficient liquid acquisition and transport. Void volume, asused herein, is to be calculated as the reciprocal value of the densityvalues obtained by the test method below. It should be noted that thesevalues are obtained under a weight of 270 g, resulting in a pressure of2100 Pascal (about 0.3 psi). Hence, void volume values discussed hereinare representative of the behavior of the material under pressure. Apressure of 2100 Pascal is thought to be generally representative ofin-use conditions and in many cases corresponds to the pressure exertedby a baby onto a diaper when the baby is sitting.

In a further important aspect, in accordance with the present invention,the respective void volume values have to be met not only at 20° C., butalso as 40° C. The temperatures which a typical liquid acquisitionmember in an absorbent article will encounter throughout his lifetimevary widely. Often, and namely when the absorbent article or the liquidacquisition or the material for the liquid acquisition member is stored,the storage temperature will be around 20° C. However, also highertemperatures will be encountered, depending of the storage and transportconditions. In one aspect, a temperature of 40° C. is thought to berepresentative of storage and transport conditions in hot climates. Inanother important aspect, a temperature of 40° C. is thought to berepresentative of the temperatures encountered when an absorbent articleis worn. It is to be noted, of course, that the temperature of theliquid acquisition member, when an absorbent article is worn, dependsboth on the body temperature of the wearer and on the ambienttemperature. For low ambient temperatures the encountered temperaturemay be below 40° C., however, given that a typical body temperature is37° or 38° C., it is believed that temperatures of 40° C. are ofrelevance also for typical wearing conditions.

When conceiving the present invention, it has been realized that itseems beneficial to use certain binders in liquid acquisition membersfor obtaining liquid acquisition members which have a void volume underpressure of at least 7 cubiccentimeter per gram, and which are able tomaintain this void volume value in a temperature range at least from 20°C. to 40° C. Without wishing to be bound by theory, the applicantbelieves that the rheology of the binder is of importance formaintaining a high void volume.

According to the present invention it is crucial that the rheology ofthe binder, as determined on a film prepared from the binder has a tan δvalue at 40° C. which is not greater than the tan δ value at 20° C.

Tan δ values are obtained in accordance with the test method describedherein. Preferred liquid acquisition layers according to the presentinvention have a tan δ value at 40° which is less than 0.6, preferablyless than 0.5, more preferably less than 0.4, and most preferably lessthan 0.3.

Without wishing to be bound by theory, it appears that tan δ valuesincreasing with temperature lead to a loss in void volume of the liquidacquisition member when the member is under an external pressure at anelevated temperature. It appears, that liquid acquisition members willoften not recover and regain their original void volume when they havebeen subjected to pressure at an elevated temperature and their tan δvalue at such a temperature is higher than the tan δ value at lowertemperatures.

Again, without wishing to be bound by theory it also seems beneficialthat films formed from binders useful for liquid acquisition membershave a certain G′-curve. In one important aspect, the G′ versustemperature curve should have an extended rubbery plateau. An extendedrubbery plateau corresponds to a relatively constant G′ value over abroad temperature range. Preferably G′ is constant over a temperaturerange of at least 0° C. to 80° C., preferably at least 0° C. to 60° C.,and most preferably at least 20° C. to 40° C. Herein, G′ is consideredconstant, if its maximum value and its minimum value over a given rangedo not differ by more than 15%, preferably not more than 10%, mostpreferably not more than 5%.

The Applicants believe that in particular the increase of thecarboxylation level as compared to prior art binders for non-wovenmaterials provides a G′-curve with an extended rubbery plateau. Binderswith an extended rubbery plateau remain elastic up to highertemperatures. Therefore, non-woven materials incorporating such binderscan yield in response to external pressure without major re-arrangementsin their fibrous structure. The Applicant believes that this contributesto their better ability to recover.

Another important aspect it is believed that binder materials mustuseful for the present invention will have a low glass transitiontemperature T_(g). The glass transition temperature, as used herein, ismeasured by differential scanning calorimetry (DSC). Preferably, T_(g)is in the range −10° C. to +10° C., more preferably −5° C. to +5° C.,and most preferably from −3° C. to +1° C. A glass transition temperatureT_(g) which is well below the expected usage and storage temperatures ofthe liquid acquisition layers helps to maintain the rubbery likebehaviour of the binder, and thereby improves the ability of the layersto recover. Because the present invention preferably employs bindershaving an extended rubbery plateau, the binders may have a low glasstransition temperature T_(g) and at the same time allow the binder toact in the rubbery plateau regime at elevated temperatures, such as 40°C., 60° C. or 80° C.

In another important aspect, a preferred material will have a goodrecovery at 20° C., a temperature which is considered representative ofroom temperature, and thereby representative of many transport andstorage conditions. A preferred material will further have good recoveryvalues over a broad range of temperatures, namely temperatures above 20°C. A temperature of 45° C. is thought to be representative of relativelyhigh temperature storage conditions, but the temperature is also thoughtto be representative for the upper end of temperatures encountered underwearing conditions. A temperature of 60° C. is thought to berepresentative of more extreme storage and transport conditions, asencountered in particularly high climates. Acquisition layers inaccordance with the present invention will not loose an substantialamount of their ability to recover from external pressure when thematerial is held as a temperature of 45° C., or 60° C., respectively.

Preferred materials according to the present invention will have arecovery at 20° C. of at least 70%, more preferably of at least 75% or80%. For preferred materials the recovery value is at 45° C. will be atleast 45%, more preferably 50%, 55% or at least 60%.

Preferably, the recovery value at 45° C. is at least 65% of the recoveryvalue at 20° C. More preferred are higher recovery values, namely of70%, more preferably 75%, yet more preferably 80% of the recovery valueat 20° C. Preferred material will also still at 60° C. have recoveryvalues which are high as compared to their recovery values at 20° C.,preferably the recovery value at 60° C. is at least 50%, more preferably55%, yet more preferably 60% or 65% of the recovery value at 20° C.

The recovery values of materials useful for absorbent articles, and inthe present context of a liquid acquisition member of such absorbentarticles are relevant at different pressures. For example, a sittingbaby will exert a pressure of about 2.1 kPa (about 0.3 psi), however,the pressure very much depending on the baby wearing the diaper and onthe circumstances of its sitting. When absorbent articles, for examplediapers, are shipped in plastic packaging, they are often subjected topressures in the range of 6 to 10 kPa (about 0.9 to 1.5 psi). Whenmaterials useful for liquid acquisition layers are shipped in roll stockform, they maybe subjected to pressures up to 50 kPa (about 7 psi).Without whishing to be bound by theory it is believed that the recoverybehaviour of a material under very high pressures is representative alsofor the recovery value at lower pressures. Therefore, it is believedthat a material which provides excellent recovery performance under highpressures as experienced for example by roll stock transport, will alsoguarantee excellent recovery behaviour when taken out of a package andwhen being worn. Therefore, the measurements of the recovery values asdescribed below are carried out under a pressure of 50 kPa.

Liquid acquisition members in accordance with the present inventioncomprise a multitude of fibers, preferably in the form of a non-wovenmaterial and a binder. While a large variety of fibers and binders aresuitable, it has been found that certain materials provide a betterrecovery than other materials and are therefore preferred for materialsaccording to the present invention.

Preferred fibers according to the present invention are polyesterfibers, such as PET fibers. The acquisition materials have been found towork best if a blend of different fibers is used. While a blend of 3, 4,5 or more different fibers can be used, preferably a blend of two fibersis used. Such blend may comprise at least 10, 20, 30, 40, 50, 60, 70,80, or 90 weight % of a first type and the remaining weight % portion ofa second type of fiber. Highly preferred according to the presentinvention is a 50:50 blend of fibers, i.e. 50 weight percent of a firsttype of fibers is blended with 50 weight percent of a second type offibers.

Preferably, at least the second type of fibers will exhibit aspiral-crimp. As used herein, a spiral-crimp is any three-dimensionalcrimp and preferably one wherein the fibers substantially assume ahelical shape.

Optionally, the first type of fibers may also be spiral-crimped. In oneembodiment of the present invention the first type of fibers and thesecond type of fibers are identical.

The first type of fibers may be thinner than the second type of fibers.Preferably the first type of fibers will have 3-9, more preferably 5-8,and most preferably 6-7 dtex. The second type of fibers will preferablehave 8-12, more preferably 9-10 dtex. The first type of fibers and thesecond type of fibers may be of different length, but preferably are ofthe same length. Preferably, the fibers have an average length from20-70 mm, more preferably from 30-50 mm. The first type of fibers willpreferably have a higher crimp value as the second type of fibers.Preferred crimp values for the first type of fibers are 8-12 crimps perinch (cpi), and more preferably 9-10 cpi. For the second type of fibers4-8 cpi are preferred, and 5-7 cpi are even more preferable.

A preferred first type of fibers can be obtained from Wellman, Inc.Ireland under the trade name H1311. A preferred second type of fiberscan be obtained from Wellman, Inc. under the trade name T5974.

One preferred type of fibers useful in the present invention areso-called bi-component fibers, where individual fibers are provided fromdifferent materials, usually a first and a second polymeric material.The two materials may be chemically different (hence the fibers arechemically heterogeneous) or they may differ only in their physicalproperties while being chemically identical (hence the fibers arechemically homogeneous). For example, may the intrinsic viscosity of thetwo materials be different, which has been found to influence thecrimping behaviour of the bi-component fibers. Hence, chemicallyheterogeneous bi-component fibers and chemically homogeneousbi-component fibers are preferred in accordance with the presentinvention.

Bi-component fibers, which are especially suitable for the second typeof fibers are side-by-side bi-component fibers as disclosed for examplein WO 99/00098. A specifically preferred type of bi-component fiber is afiber of circular cross section with a hollow space in the centre. It ispreferred that 10-15% of the cross sectional area are hollow, morepreferably 20-30% of the cross sectional area are hollow.

Preferably, at least one type of fibers and preferably two or more typesof fibers are crimped. For the first type of fibers a 2D crimp or “flatcrimp” is preferred. For the second type of fibers a 3D or spiral-crimpis preferred. It is believed that the use of side-by-side bi-componentfibers is beneficial for imparting a spiral-crimp to the fibers.

Without wishing to be bound by theory, it is further believed that thespiral crimping of fibers is very beneficial for their liquidacquisition and distribution behaviour. It is assumed that the spiralcrimp increases the void space in an acquisition member formed by suchfibers. Often, an absorbent article, when being worn, is exposed to acertain pressure exerted by the wearer, which potentially decreases thevoid space in the acquisition member. Having good permeability andsufficient void space available are important for good liquiddistribution and transport. It is further believed that the bi-componentspiral-crimped fibers as described above are very suitable to maintainsufficient void volume even when an acquisition member is exposed topressure. Also are spiral-crimped fibers believed to provide for goodpermeability as for a given fiber dtex value, the hollow fibercross-section allows for a larger outer diameter of the fiber ascompared to a compact cross-section. The outer diameter of a fiberappears to determine the permeability behavior of an acquisition memberformed by such fibers.

While any of the above-described manufacturing processes for non-wovensare suitable to provide an acquisition member in accordance with thepresent invention, a highly preferred manufacturing process for such anacquisition member in both carding resin-bond technology as described inWO 98/22279. As part of this process the binder will be cured and driedafter having been applied to the web.

Nonwoven materials having a basis weight from 20 to 100, preferably from30 to 80, and most preferably from 50 to 70 gram per square meter arepreferred. Basis weight is determined in accordance with the test methodbelow using a 100 cm long sample of a width given by the role width (butat least 10 cm).

The materials of the present invention are preferably stabilized by alatex binder, and most preferably a styrene-butadiene latex binder (SBlatex). Processes for obtaining such lattices are known, for example,from EP 149 880 (Kwok) and US 2003/0105190 (Diehl et al.). Preferred SBlattices, however, are obtained in using more than 10 weight % of amono-, or bi-carboxylic acid, and will herein be referred to as having acarboxyllation level of more than 10%. Preferred SB latices will have acarboxyllation level from 10% to 25%, more preferably from 10% to 20%.Depending on the ultimate application of the binder level in excess of12%, 14% or 16% are preferred. A particularly preferred SB latex isavailable under the trade name GENFLO™ 3160 (OMNOVA Solutions Inc.;Akron, Ohio).

Without being bound by theory, it is believed that such binders are moreeffective because the temperature sensitivity of the recovery value forsuch binders is reduced compared to other binders of the prior art. Asis well known, latex binders are applied to a nonwoven structure in aliquid form that accumulates in fiber intersections. After curing, thebinder forms a coherent polymeric film that stabilizes the nonwovenstructure. When the nonwoven is stressed (e. g. by compression), thepolymeric film resists that stress so the fibers comprising the nonwovenare inhibited from moving. If the stress causes the polymeric film toextend past its yield value the fibers can move irreversibly withresulting reduced recovery. The Applicants believe that the recoveryvalue of the liquid acquisition layers of the present invention is lesstemperature sensitive because the employed binders have improvedrheology, namely an improved elastic behaviour, at elevatedtemperatures. Therefore, the liquid acquisition layers of the presentinvention can recover from applied stresses instead of yielding withirrevocable fiber movement.

TEST METHODS Density/Caliper/Basis Weight Measurement

A specimen of a defined area such as by cutting with a sample cutter isweighed to at least 0.1% accuracy. If not stated otherwise, caliper ismeasured under an applied pressure of 2.1 kPa (0.3 psi) by using aconventional caliper measurement device with a flat plate with adiameter of 2 cm, which can be loaded with defined weights. The testspecimen can then be placed between this plate and a flat surface andthe distance between the plate and the base surface can be measured. Thestandard caliper measurement is executed by carefully (to avoid overcompression) applying a weight of 270 g. The weight is left for at leastabout 5 seconds, upon which the distance reading is taken.

This procedure is repeated at least three times for one specimen toprovide a representative number of test data.

The basis weight of a test specimen can be tested by determiningsufficiently accurately the weight of a test specimen of known area.Conveniently, a test specimen of 10 cm by 10 cm is weighted e.g. on ascale having an accuracy of 0.001 g.

From basis weight as weight per unit area expressed in g/m2 and caliperexpressed in mm at 2100 Pa pressure the density expressed in g/cm3 canbe readily calculated.

Recovery Value Measurement

Equipment: Dynamic Mechanical Analyzer 2980 in combination with a GasCooling Accessory (GCA) both provided by TA Instruments, USA orequivalent. The GCA uses liquid Nitrogen, which is evaporated and ledthrough the furnace of the DMA in order to control the temperature incombination with the heater of the furnace and to provide inertmeasurement atmosphere. The Isothermal stability should not be less than+/−1° C.

To work in compression mode a standard compression clamp provided by TAInstruments with compression plate diameter of 15 mm is employed. Themaximum compression force of the instrument should be 18N or more. Theoptical encoder (position sensor to determine the position of the driveshaft, which is connected to the moveable upper compression plate)delivers a strain resolution of 1 nm.

Procedure:

Sample Preparation:

Using the punch provided by TA Instruments a cylindrical sample of 13 mmdiameter is punched out of the non-woven sample (NW) and placed onto thelower plate of the compression clamp. To determine the initial thicknessof the NW, the upper plate is placed carefully above the NW by hand. Theupper plate should not touch the NW in order to prevent uncontrolledpre-compression. After the upper plate is positioned appropriately, themotor drive shaft is blocked to keep the upper plate in position.

DMA—Compression Creep Test Method:

The Compression Creep Test consists of a compression step, where acertain stress is applied to the NW, and a subsequent recovery step,where the stress is set to zero in order to determine the resiliency orrecovery of the NW.

The following method parameters are defined:

Temperature: The temperature is kept constant for the entire experiment.Experiments are typically conducted at either 20° C., 45° C. or 60° C.in order to determine temperature dependence of the recovery functions.(One experiment per temperature option.) Equilibration An equilibrationtime of at least 10 min has to be time: programmed ahead the compressionstep in order to allow the NW and the instrument to get in thermalequilibrium Poisson 0.44 ratio: Preload 0.02 N, applied during theentire experiment to keep force: upper compression clamp plate incontact with NW-sample after method start Sample Cylindrical:measurements: Diameter: 13 mm, determined by punch Thickness: To ensuredefined pre-compression prior to the determination of the samplethickness for all experiments a force of 0.02 N is applied for 5 secondsto the upper plate driving the upper plate against the NW. (Start methodand cancel after 5 seconds blocking the drive shaft after cancellation.The compression should come to equilibrium within 20 micrometer oftravel of the upper plate.). An equilibrium thickness is achieved. Thedistance between the lower and upper clamp plates determines the samplethickness (digital calliper, 2 digits). Compression : 120 min step timeStress 50 kPa (about 7 psi) applied: The DMA uses clamp correctionfactors in order to count for transverse strains due to dilatationeffects in compression mode. The stress, which needs to be entered toreceive an actual stress on the material of 50 kPa (about 7 psi) dependson sample thickness considering the clamp factors. This stress iscalculated multiplying 50 kPa (about 7 psi) with the respective clampfactor at given sample thickness (table including sample thicknessversus clamp factors provided by TA Instruments). The clamp factors haveto be interpolated in order to match sample thickness. Recovery 120 minstep time: Stress 0 Pa applied:Report:

Recovery value in % relative to the initial sample thickness ((recoveredsample thickness/initial sample thickness)×100) is plotted againstrecovery time. The final recovery value after 120 min is reported asfunction of measurement temperature.

Tan δ Measurements

The tan δ value of a binder comprising a dispersion of a polymersubstance in an essentially aqueous medium is measured on a polymer filmprepared from said binder. For this, about 2% of Paragum 147 thickenerare added and the dispersion is throroughly mixed and allowed to restfor about 15 min. Then the film is drawn with the help of a Gardnerknife and the essentially aqueous medium is evaporated in a through-airoven at 50° C. and allowed to further dry until becoming clear overnight. Subsequent curing is conducted at 150° C. for 60 seconds.

The thickness of the respective film is not critical. Measurements canvery well be performed on films having a thickness of 0.3 mm.

The tan δ is a rheology value well-known in the art, which can bereadily measured with a variety of equipments. While tan δ correspondsto the viscous modulus G″ divided by the elastic modulus G′, the tan δvalue can also be obtained without first obtaining G′ and G″ by usingstandard rheology equipment. A Mechanical Analyzer 2980 in combinationwith a Gas Cooling Accessory, both provided by TA Instruments, USAmachine has been found to work well for such measurements.

Tan δ values, as used herein art those values measured in tensile mode.

All documents cited in the Detailed Description of the Invention are, inrelevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. A liquid acquisition member for an absorbent article, comprising: amultitude of polyester fibers and a latex binder, said liquidacquisition member having a basis weight of between 30 and 80 grams persquare meter and comprising about 20 to 40 weight percent of said binderand about 60 to 80 weight percent of said polyester fibers, wherein saidpolyester fibers comprise about 20 to 80 weight percent of a first typeof fibers, about 20 to 80 weight percent of a second type of fibers, anda third type of fibers, the first type of fibers comprising flat-crimpfibers having a dtex of between about 3 and 9 and about 8-12 crimps perinch, the second type of fibers comprising flat-crimp fibers having adtex of between about 8 and 12 and about 4-8 crimps per inch, and thethird type of fibers comprising spiral-crimp fibers; and wherein saidbinder comprises a dispersion of a polymer substance in an essentiallyaqueous medium, said polymer substance being capable of forming a film,said film having a tan 6 value at 20° C. and at 40° C., wherein said tan6 value at 40° C. is not greater than said tan 6 value at 20° C.
 2. Theliquid acquisition member according to claim 1 wherein said tan δ valueat 40° C. is less than about 0.6.
 3. The liquid acquisition memberaccording to claim 1, wherein the binder comprises a styrene-butadienelatex binder.
 4. The liquid acquisition member according to claim 1,wherein the polyester fibers are carded to form a nonwoven.
 5. Theabsorbent article of claim 1, wherein the backsheet is substantiallyimpervious to liquids and comprises a laminate of a nonwoven and a film.6. The absorbent article of claim 5, wherein the film has a thickness ofbetween about 0.012 mm and about 0.051 mm.
 7. An absorbent articleaccording to claim 5, wherein said absorbent article further comprises alayer of chemically stiffened cellulose fibers.
 8. An absorbent articleaccording to claim 6, where said article further comprises a topsheetand an absorbent core, the topsheet facing the wearer when said articleis in the intended wearing position, said acquisition member and saidlayer of chemically stiffened cellulose fibers being positioned betweenthe topsheet and the absorbent core.
 9. The liquid acquisition memberaccording to claim 1, wherein said latex binder has a T_(g) of between−10° C. to +10° C.
 10. A disposable absorbent article comprising atopsheet, a backsheet, an absorbent core disposed between the topsheetand backsheet, and an acquisition member disposed between the absorbentcore and the topsheet, the acquisition member comprising: a liquidacquisition member for an absorbent article, comprising: a multitude ofpolyester fibers and a latex binder, said liquid acquisition memberhaving a basis weight of between 30 and 80 grams per square meter andcomprising about 20 to 40 weight percent of said binder and about 60 to80 weight percent of said polyester fibers, wherein said polyesterfibers comprise about 20 to 80 weight percent of a first type of fibers,about 20 to 80 weight percent of a second type of fibers, and a thirdtype of fibers the first type of fibers comprising flat-crimp fibershaving a dtex of between about 3 and 9 and about 8-12 crimps per inch,the second type of fibers comprising flat-crimp fibers having a dtex ofbetween about 8 and 12 and about 4-8 crimps per inch and the third typeof fibers comprising spiral-crimp fibers; and wherein said bindercomprises a dispersion of a polymer substance in an essentially aqueousmedium, said polymer substance being capable of forming a film, saidfilm having a tan 6 value at 20° C. and at 40° C., wherein said tan 6value at 40° C. is not greater than said tan 6 value at 20° C.
 11. Theabsorbent article of claim 1, further comprising a fastening systemconfigured to join a front waist region portion to a rear waist regionportion to form a leg opening.
 12. The absorbent article of claim 11,wherein the fastening system is reclosable.
 13. The absorbent article ofclaim 12, wherein the absorbent article is a pant-type diaper.